1. Field of the Invention
This invention relates generally to superconducting quantum interference devices known as interferometers which have a nonlinear threshold or switching characteristic. It further relates to interferometers which, as a result of their nonlinear gain characteristics, are capable of providing greater amplification than that attainable using prior art interferometers. More specifically, it relates to interferometer circuits having nonlinear gain characteristics which may be tailored in such a way as to provide logic outputs such as AND, OR. In a preferred basic amplifier circuit, a two-junction interferometer can achieve high gain by injecting current into the interferometer at a point which is different from that where the normal gate current is applied to an interferometer. The injection current is applied at the same time as the gate current and, in a complex way, provides a nonlinear switching or threshold characteristic which, relative to known interferometers, provides a greater gain.
In another embodiment, greater amplification can be achieved by magnetic field enhancement. Magnetic field enhancement is achieved by injecting current at a point different from that where the gate current is applied and simultaneously electromagnetically coupling a portion of the injected current to the inductance of the interferometer. In this manner, gains of up to ten can be achieved.
In other embodiments, AND and OR circuits can be obtained by tailoring the gain characteristic so that substantially equal magnitude inputs produce the desired logic output.
While two and three-junction interferometers are shown, it should be appreciated that any multi-junction interferometer may also be utilized. Further, there is no limitation on the number of currents which may be injected into an interferometer at points other than that to which the gate current is applied. In all events, whether the injected current be a directly injected or one which is electromagnetically coupled to the interferometer, all the means for producing a nonlinear gain characteristic are directly connected to the interferometer.
2. Description of the Prior Art
Josephson junction devices are well-known in the prior art where they are utilized both as memory devices in storage arrays and as switching devices in ultrahigh-speed logic circuits.
The injection of current into Josephson junction devices to produce a substantially linear switching or threshold characteristic is shown in U.S. Pat. No. 3,281,609 filed Jan. 17,1964, Ser. No. 338,467, in the name of J. M. Rowell. The patent shows logic gates which perform AND and OR functions using a current which is similar to the gate current of the present application and an injection current similar to that utilized in the present application. This reference can be distinguished over in that the present application provides circuit arrangements which have nonlinear threshold characteristics. The threshold characteristics of Rowell are substantially linear in character.
An article entitled "A Josephson Logic Design Employing Current Switched Junction" by T. A. Fulton et al. in the IEEE Transactions on Magnetics, Vol. MAG-13, No. 1, January 1977, p. 56 shows an application of the linear current injection scheme of the above patent.
An article in Applied Physics Letters, Vol. 19, No. 11, p. 469, Dec. 1, 1971, entitled "Josephson Junction Amplifier" by J. Clarke et al. shows a double junction interferometer which achieves current amplification using positive feedback. In the arrangement shown, a bias current is fed asymmetrically to the junctions of the interferometer. A magnetic flux is applied to the interferometer either from an external source or alternatively by means of a current in the upper arm of the interferometer. While this current could be said to be an injected current, this is not so because it is strictly confined to the upper arm of the interferometer to supply a magnetic field. This becomes clear when it is observed that the current applied to the one side of the upper arm of the interferometer is withdrawn from the upper arm of the interferometer at the opposite side of the upper arm. All this amounts to is an expedient for applying a magnetic field to the interferometer inducing a flux into the interferometer ring circuit. The resulting threshold characteristic would be linear in character.
IBM Technical Disclosure Bulletin, Vol. 15, No. 5, October 1972, p. 1604, in an article entitled "Shaping the Gate Current Versus the Control Current Characteristics of Nonlinear Josephson Gates" by F. Basavaiah shows the application of a gate current and a control signal to a Josephson gate. A portion of the gate current is fed back antiparallel to the direction of the gate current. By introducing the feedback, the slope as well as the shape of the threshold characteristic can be significantly changed. Even though the slope as well as the general shape of the threshold characteristic are changed, the relationship between the gate and control current is still preserved. The circuit uses no directly injected or coupled current other than a gate current.
IBM Technical Disclosure Bulletin, Vol. 19, No. 3, August 1976, p. 1096 in an article entitled "Double Flux Quantum Cell Without Bias Line" by W. Anacker et al. there is shown an interferometer which in FIG. 2 appears to schematically show an injected current. This is not an injected current but rather is a schematic representation for a control current being coupled to the interferometer.
Swiss Pat. No. 580,365 issued Aug. 15, 1976 and assigned to the same assignee as the present invention shows a single Josephson junction into which a plurality of currents are introduced. To the extent that these currents could be said to be injected into a junction, they are all summed in the same manner as if an interferometer were fed with a single gate current which is the sum of all the applied currents. In addition, for the logic circuits shown all the current supplied to the logic circuits are applied linearly such that I.sub.Bo + I.sub.A + I.sub.B .gtoreq. I.sub.mo. The arrangements shown exhibit no nonlinearities.
An article in Applied Physics Letters, Vol. 20, No. 11, June 1, 1972, p. 456, entitled "Three-Josephson-Junction Interferometer" by D. L. Steuhm et al. shows an asymmetrically fed three-Josephson-junction interferometer to which a magnetic field is applied in the usual manner. Such arrangements do not exhibit linear threshold characteristics similar to those of the present application because there is no injected current in addition to the gate current. The latter is asymmetrically introduced in the reference.
As can be seen from the foregoing, the use of interferometers with control lines and the injection of current into single junctions is well-known. However, none of the arrangements of the prior art produces a nonlinear threshold or switching characteristic similar to that provided by the interferometers of the present application. It thus appears that the prior art is limited in the amplification mode to relatively low gains as compared with the arrangements of the present application. It further appears that all known interferometers utilize a control line to accomplish both switching and logic functions. The present invention eliminates the need for any control lines.